Many polymeric materials are foamed to provide low density articles such as films, cups, food trays, decorative ribbons, and furniture parts. For example, polystyrene beads containing low boiling hydrocarbons such as pentane are formed into lightweight foamed cups for hot drinks such as coffee, tea, hot chocolate, and the like. Polypropylene can be extruded in the presence of blowing agents such as nitrogen or carbon dioxide gas to provide decorative films and ribbons for package wrappings. Also, polypropylene can be injection molded in the presence of these blowing agents to form lightweight furniture parts such as table legs and to form lightweight chairs.
Polyesters such as poly(ethylene terephthalate) have a much higher density (e.g. about 1.3 g/cc) than other polymers. Therefore, it would be desirable to be able to foam polyester materials to decrease their weight for use in making molded parts, films, sheets, food trays, and the like. Such foamed articles also have better insulating properties than unfoamed parts. However, it is difficult to foam such polyester materials because of the low melt viscosity and low melt strength of typical poly(ethylene terephthalate) and related polyester polymers. The low melt viscosity and low melt strength of the polyesters is a problem because the polymer melt will not adequately retain the bubbles of an expanding gas during molding or extrusion operations. It would be desirable therefore to be able to provide polyester polymers which could be foamed with conventional foaming systems.
One approach to providing polyesters having high melt viscosities involves treating preformed polyesters with monomeric materials which are multifunctional carboxylic acids or polyols to provide branched polyesters. Such compositions are disclosed in U.S. Pat. Nos. 4,132,707; 4,145,466; 4,999,388; 5,000,991; 5,110,844; 5,128,383; and 5,134,028. The branching agents used include tri- and tetracarboxylic acids and anhydrides such as trimesic acid, pyromellitic acid, and pyromellitic dianhydride or polyols such as trimethylolpropane and pentaerythritol. These monomeric branching agents will provide polyesters with increased melt viscosities and melt strengths but their use is often disadvantageous. The usual method of adding the branching agent is to melt the polyester in an extruder and to add the branching agent into the melt in the barrel of the extruder. However, it is difficult to control the amount of branching agent used and to get adequate mixing and reaction before the melt exits the die.
To resolve the aforementioned problems and provide polyesters with sufficient melt strength that are easy to foam and result in excellent foamed properties, one of the inventors of the present invention disclosed in U.S. Pat. No. 5,399,595 the incorporation of small amounts of a dicarboxylic acid sulfomonomer such as sulfoisophthalic acid in a polyester composition of essentially repeat units from terephthalic or naphthalenedicarboxylic acid and an aliphatic or cycloaliphatic glycol. Such foamable polyesters are prepared by a combination of melt phase and solid state polymerization to result in a polyester composition that is ready to foam.
In an effort to control the amount of branching agent added to a polyester to improve foaming results, the inventors disclosed in U.S. patent application Ser. No. 130,816 filed Oct. 4, 1993 a branching agent concentrate that may be conveniently dry blended with polyesters, melt compounded and foamed using conventional foaming techniques. The branching agent concentrate is a melt blended composition containing a polycarboxylic acid or polyol branching agent having at least three carboxylic or hydroxyl groups and a polyolefin wherein the polyolefin acts as a physical carrier for the branching agent.